Explosion of Crater Lake in the “Cameroon line” area; seismic contribution CTBT:S&T2011 T1-P35 Parfait N. ELOUMALA ONANA Sciences faculty of Ibn Tofail University, P.O. Box 133 Kenitra, Morocco Introduction Lakes Monoun and Nyos Seismicity In the Gulf of Guinea (Africa), the “Cameroon volcanic line” is geologically Data used have been collected from temporary seismic network (Ateba et al., 2009). unusual in extending through both the ocean and the continental crust. In the inland, there are crater lakes, of which two have already exploded in a limnic eruption which resulted in the release of a large amount of carbon dioxide coming from volcanic activity: - Lake Monoun in 1984 and causes such as terrorism were suspected. - And then the Lake Nyos in 1986. The seismicity of “Cameroun line” reveals that many of the tectonic phenomena follow the previous structural lines and different clusters of epicenters are identified. Although the area has a relatively low seismic activity, the implication of earthquake is considerable in the occurrence of lake explosions; specially, shaking the crater lakes which explodes and releases the gas. By these natural events, we want to make known the area Figure 2: Lake Monoun (Altitude: 900m, Depth: 100 m ) of “Cameroon line” which is a propitious site of diversion. - Lake Monoun lies in the Oku Volcanic Field, it exploded in 1984 and released of a large amount of carbon dioxide that killed 37 people” in nearby villages. Figure 6 : Vertical profile of hypocenter along N40°E, the a xis of the volcano (a) and along Figure 5 : Seismicity of volcanic field. The first striking result is the clustering of epicenters N130°E, the direction perpendicular to it (b). Posi tions are given on fig.5 (Seismic activity is marked by seismic swarms underneath the Lake crater.) An interesting aspect of Cameroon line seismicity is the regular occurrence of deep, near-constant magnitude, low-frequency microearthquakes near the volcano. Both profiles depict a vertical linear feature from the surface to a depth of 20 km under the lake region. We believe that this result likely suggests the existence of a vertical magma conduit under the central axis of the volcano from the surface to about a 20 km depth. Figure 3: Lake Nyos (Crater Lake: 1.58 km 2, Altitude: 900 m, depth: 200 m) . - The Lake Nyos, in the Oku volcanic plain suddenly exploded on August 21, 1986, and emitted a large cloud of CO2, which suffocated 1,700 people and 3,500 livestock within 25 km radius. Both Lakes Nyos and Monoun are very deep and sit in a valley where there is little wind, so the water in the lake is still and doesn’t get mixed up. Figure 7: Frequecny-magnitude relation for year 2000 Figure 8 : Seismic energy release in 2000 Figure 1: Location of the main volcanic centers belonging to the Cameroon Volcanic Line. The b-value importantly varies in time: The first b-value based on data collected in 1987 are lower (0.18 – 0.86), using 1999 The Cameroon Volcanic Line forms a part of the North Equatorial Pan-African dataset, b-value is 0.55+0.01, with 2000 dataset we found b-value of 1.43±0.02. Although the seismicity is moderate, the b- chain. It is a zone of volcanic centers that trends N 30° and extends from value expresses the higher stress condition from time to time. In 1999 for example, this is supported by occurrence of many Island of Pagalù in the Gulf of Guinea to Lake Chad. felt events with 4-5 magnitudes. Thus we assume that in 1984 and in 1986, felt events with important magnitude have shaken lakes Monoun and Nyos respectively. The Build -Up of Gas Conclusion The science community has come to an agreement that the origin of CO2 within Lake Nyos is due to CO2 that rises from volcanic activity. This Figure 4: sketch of stratified lakes and degassing process We think that the earthquakes that shook the Oku volcanic plain led Large rocks to fall into the lake and caused some of the CO2 is then dissolved into groundwaters and transferred to the lake resulting cold, CO2-rich water to rise to the surface suddenly mixing up Monoun (in 1984) and Nyos (in 1986) lakes. The Lakes in the slow saturation of the hypolimnion. The lake is compositionally stratified, with cold, heavy water sits at the bottom, exploded and released the gas like a soda. and warm, light water floats above it. In most crater lakes, turnover of the stratified waters occurs periodically and By these natural events, we want to make known the area of “Cameroon line” which becomes a propitious site of diversion harmless amounts of dissolved gases are released; however, these two The lake is now degassed in a controlled way to prevent a reoccurrence because of previous occurrences. In prospect, we suggest a permanent monitoring to characterize each event and then to be (Halbwachs, M., 2001 ). particular lakes do not periodically turn over (Holloway, 2000). Thus, able to distinguish a potential man-made event. dissolved gases are allowed to reach much higher concentrations. Ateba B., Dorbath C., Dorbath L., Ntepe N., Frogneux M., Aka F.T., Hell J.V., Delmond J.C., Manguelle D.,2009, Eruptive and earthquake activities related to 2000 eruption of Mont Cameroon volcano. J. Volcanol. Geotherm. Res. 179, 206-216 Halbwachs, M., 2001, Degassing Nyos, available at http://perso.wanadoo.fr/mhalb/ nyos/2000/index2000.htm. Holloway, M., 2000, The Killing Lakes: Scientific American, v. 283, p. 92-99. .